Canon LTM Canon 7 light leaks

Canon M39 M39 screw mount bodies/lenses
I think the back door might be loose?

If you want to have a go, put some black tape all around the edges of the door when you shoot your next roll.
 
Check the seals around the door hinge. The fact that the leak is affecting the middle of the frame means its not likely happening through the film gate. The leak could be happening after the film has been exposed and advanced to the next frame.
 
Definitely a door hinge leak. If you look at the whole roll, you'll see it kind of walks across the frames. That is because as you wind the film on the take-up spool, it gets larger in diameter, and with each rotation the end of the frame winds up a little bit further from the last stopping point.

It's also a typical door leak as on your color shots, there is red in the color of the light strike. This is due to the light shining on to the back of the film through the aniti-halation layer, thus giving more exposure to the red layer in the emulsion (closest to the back of the film). It's an effect called Redscale some folks (myself included) use to their advantage by rolling film backwards onto the cassette, and exposing it at a much lower ISO (for 400 ISO film, use 25 or 50 ISO).

A complete seal replacement is called for. If you feel competent enough, you can do it yourself with a kit from Jon Goodman who also includes complete instructions.

One other thing to think about, carefully shade the camera while changing lenses when you are out shooting, making sure not to point the camera towards the light, as it will work it's way around the shutter curtain top and bottom.

PF
 
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